While yield determination in soybean can be reduced down to this simple concept, the factors affecting crop growth and photosynthate production during these long reproductive periods are extremely complex. Maximizing yield depends upon alleviating all stresses throughout the entirety of reproductive development.
Prior to flowering, stresses do not have a large impact on final yield, provided that the stress did not severely stunt the plants. It is critical to close the rows and have >95% light interception by the beginning of flowering to maximize photosynthate production. This will often require a row spacing <30 inches.
Early planting and/or warm spring temperatures can induce earlier flowering and extend the flowering period. Soybeans initiate flowering based upon the interaction of daylength and heat unit accumulation (Lawn and James, 2011). Shorter daylengths hasten development. This can be observed with research from southern Pennsylvania in 2015 where shorter daylengths with both very early and very late planting reduced the growing degree units (GDUs) required to reach R1 (Table 1).
Table 1. The duration and GDU accumulation of vegetative and reproductive growth periods for soybean maturities Late II to Mid III from Parker et al. (2016).
Parker et al. (2016) showed that soybean planted on April 15 would begin flowering on June 4, while May 11 planted soybean did not flower until June 30 after the summer solstice and when days begin to shorten (Figure 4). Longer daylengths extend reproductive development. This is evident by the longer R1 to R6 period with earlier planting (Table 1; Figure 4). Greater GDU and solar radiation accumulation from R1 to R6 have the potential to increase photosynthate production and thus, seed number and yield.
Figure 4. Daylengths during the spring and summer growing season for Pittsburg, PA, (40.4° N) with example planting, R1, and R6 dates from Parker et al. (2016).
Full season varieties can have longer vegetative and reproductive periods and often have greater yield potential when planted early compared to short season varieties. In the Mid-South, short season varieties can be planted too early and have later optimum planting dates compared to fuller season varieties (Poston and Jeschke, 2015; Salmerόn et al., 2016). Further north, short season varieties can be planted later with less associated yield loss (Nafziger and Vossenkemper, 2015). In both scenarios, planting full season varieties first will maximize yield potential.
After planting, growing conditions throughout reproductive development will have the greatest influence on final yield. Both too much and not enough water can have a large impact on photosynthesis and crop growth. Fertility and pH must also allow for optimal crop growth rates. This should be managed according to soil and plant analyses in conjunction with the yield goal and calculated crop demands.
While weed control is important, herbicide applications during reproductive growth should be avoided due to the potential for reduced photosynthate production and seed number. This is especially true if the herbicide causes plant injury (Kyle, 2014). In-season management of insects and diseases are also crucial to limit their impact on crop growth and yield.
All of the aforementioned practices and any others should focus on achieving optimal growing conditions from R1 through R7 to maximize photosynthate production during seed set and to lengthen the seed fill period in order to maximize yield.